A copending U.S. patent application Ser. No. 281,655 filed July 9, 1981 describes a new and improved pheresis process and apparatus generally constructed as follows. A first container, in the form of a flexible bag containing anticoagulated whole blood to be centrifugally separated, is located on a centrifuge rotor a suitable distance away from the center of rotation of the rotor. A second container is disposed adjacent the first container and in fluid communication with the first container. The second container, which also may be a flexible bag, is adapted to receive one or more of the centrifugally separated components of the anticoagulated whole blood.
A pressure plate in the form of a body of material, such as a metal plate, having a predetermined mass is disposed between the first bag and the center of rotation of the rotor. This pressure plate is suspended so that it is free to move radially against the first bag when subjected to the centrifugal forces generated by rotation of the centrifuge. The pressure plate has a predetermined mass sufficient to at least initiate a flow of separated fluid component from the first bag to the second bag as the pressure plate presses against the first bag during rotation of the centrifuge rotor.
The pressure plate has a predetermined mass distribution and shape adapted to pool the separated first blood component in the area of the output of the fluid communication to the second bag. The pressure plate is adapted to press against the first bag and cause the radius at the output of the first bag to be located at the minimum radius of the first bag in the centrifuge.
The first bag and second bag are located adjacent each other on the rotor with the first bag positioned radially inward from the second bag. A siphon effect is created when flow is initiated from the first bag to the second bag as the pressure plate pushes against the first bag under the influence of centrifugal force. The siphon effect is due to the difference in centrifugal forces to which the bags are subjected because one bag is located nearer the center of rotation than the other.
Flow from the first bag to the second bag, once initiated, continues regardless of the specific gravity of the separated blood component. Therefore, a valve is provided in accordance with copending U.S. patent application, Ser. No. 281,649 filed July 9, 1981. This valve may be in the form of a stopper, such as a ball, having a specific gravity less than the component or components to be retained in the first bag, but greater than the component or components to be expressed into the second bag.
Prior to the start of the pheresis procedure, a sufficient volume of anticoagulant may be stored in the first bag. Alternatively, the anticoagulant may be mixed with the whole blood as it is drawn from the donor and passes into the first bag.
After the whole blood is collected in the first bag; the first and second bags with appropriate interconnections (and optionally additional bags if further separation is required) are loaded into a cassette, such as a free standing rack partitioned into a number of vertically extending annular sections (one for each bag) having a shape corresponding to a segment of a cylinder with a radius corresponding to the radius to the center of rotation of the centrifuge rotor.
The centrifuge is then brought to a suitable speed, for example 2000 r.p.m., for a sufficient time to achieve centrifugal separation of blood components within the first bag. During this separation time or "dwell period", the conduit between the first and second bag is closed off by suitable means, such as the timing mechanism described in copending U.S. patent application, Ser. No. 281,650 filed July 9, 1981.
After a sufficient dwell period has elapsed, the conduit between bags is opened by the timing mechanism to allow flow of separated component, such as plasma, from the first bag to enter the second bag.
During the dwell period, the pressure to which the bags are subjected is considerable and may be calculated as follows: EQU P=1/2.rho..omega..sup.2 [r.sub.o.sup.2 -r.sub.i.sup.2 ]
where:
P=pressure in dynes per cm.sup.2. PA1 .rho.=density of blood in grams per cm.sup.3. PA1 .omega.=rotating velocity in radians per second. PA1 r.sub.o =outside radius of the bag in cm. PA1 r.sub.i =inside radius of the bag in cm.
The pressure in dynes per cm.sup.2 is converted to pounds per square inch (psi) by multiplying by 1.45.times.10.sup.-5. In a typical embodiment of the pheresis apparatus described in the above referenced patent application Ser. No. 281,655, the pressure on the first bag is about 59 psi based on a 2000 r.p.m. dwell period rotor speed, a .rho. of 1.1 gm/cm.sup.3, and wherein r.sub.i =0 and r.sub.o =13 cm.
Commonly, the first bag is manufactured from two sheets of PVC welded together at the edges. The welds and material of an unsupported thin-wall common PVC blood bag will not withstand 59 psi for even a short period of time. Yet it is desirable to employ inexpensive construction and materials in the fabrication of the bags constituting the pheresis software set; since they are disposable items and cannot be used more than once.
Rather than go to expensive materials and construction for the bags, one might consider a support structure for the bags as described in U.S. Pat. No. 4,146,172 dated Mar. 27, 1979 to Cullis et al. In the 172' patent, a separation chamber is constructed of two sheets of PVC or other hemo-compatible plastic material bonded together to form a shaped inner compartment.
The separation chamber is carried in a pair of rectangular plates formed of thermal conductivity metal (column 8, lines 7-10). The plates have recesses forming compartments for receiving the chamber. Pressure on the chamber seams is relieved by an interior rib provided on the periphery of one recess (column 8, lines 20-26).
The rigid plates in the 172' patent are similar to the shoes in U.S. Pat. No. 4,285,464 issued to Latham, Jr. They provide a rigid recess which supports the separation bag so it can withstand high operating pressures.
Unfortunately, such a solution is not available in a pheresis system, such as above described, which relies on the movement of a weight or pressure plate against one side of a flexible bag to express separated component from a processing chamber (the flexible bag) which has a variable volume.